1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, to a device using a semiconductor element in a frequency band within a frequency region of a millimeter waveband to a terahertz waveband (30 GHz or more and 30 THz or less; hereinafter, which is used similarly), and a production method therefor.
2. Description of the Related Art
In recent years, the basic development of a semiconductor element such as the generation, amplification, and detection of an electromagnetic wave (hereinafter sometimes referred to simply as “terahertz wave”) containing at least partial frequency component of a frequency band from a millimeter waveband to a terahertz waveband has been performed actively. This is because a terahertz wave has the following properties and is considered to be useful for industry. First, the terahertz wave passes through a nonmetallic substance like an X-ray. Second, absorption spectra peculiar to biological molecules and medical products are present in a great number in the frequency band of the terahertz wave. Third, the terahertz wave has a spatial resolution required for a great number of imaging applications. From the foregoing features, as application fields of the terahertz wave, a spectroscopic analysis technology for an inside of a substance, a safe transparent imaging device replacing an X-ray, an analysis technology for biological molecules and medical products, and the like have been considered.
As a basic semiconductor element in the above-mentioned frequency region, one method is to consider an electronic device as an approach from a millimeter waveband. Known examples of the electronic device are basic semiconductor elements such as a negative resistive element for generating an electromagnetic wave, a transistor using a heterojunction for amplifying an electromagnetic wave, and a rectifying element for detecting an electromagnetic wave.
In a device using the above-mentioned semiconductor element, the presence of unintended parasitic reactance and parasitic resistance which remarkably degrade the performance on a high frequency side is one of problems. The factor for causing this problem is wiring. The parasitic electric delay caused by stray capacitance Cs in wiring up to the semiconductor element and series resistance Rs thereof needs to be small. Therefore, in general, the electric delay is reduced by miniaturizing the wiring and element layout. However, there is a limit to such a method, and hence the following method has hitherto been considered.
Japanese Patent Application Laid-Open No. 2009-152617 discloses a semiconductor device using a low-permittivity material called benzocyclobutene (BCB) having a specific permittivity of about 2 to 3 in a device using a Schottky barrier diode for a microwave and a millimeter wave. The semiconductor device disclosed in Japanese Patent Application Laid-Open No. 2009-152617 has a structure in which the stray capacitance Cs parasitic in wiring is suppressed to reduce an electric delay (RsCs). Further, Japanese Patent Application Laid-Open No. 563-122179 discloses a semiconductor device in which wiring up to a semiconductor element is bridged in a hollow, and the air (or inactive gas) having a specific permittivity of about 1 is used in the hollow portion. In such a structure, the stray capacitance Cs can be further suppressed although there is concern about electric connection at an inter-element level.
However, in Japanese Patent Application Laid-Open No. 2009-152617, there is a limit to the reduction in parasitic capacitance because a dielectric having a specific permittivity which is low but still larger than that of air is used. In Japanese Patent Application Laid-Open No. S63-122179, the semiconductor device has a structure in which the mechanical strength is industrially unstable as in yield of hollow wiring.